South Australian police have declared the disappearance of four-year-old Gus Lamont to be a “major crime” and say they believe he is dead.

Gus was last seen by his grandmother playing on a mound of dirt at the sprawling Oak Park sheep station in outback South Australia on September 27.

Detective Superintendent Darren Fielke said on Thursday that someone known to Gus, who lives at the property, was now considered a suspect.

However, Fielke stressed that this person was not Gus’s parents.

He confirmed the boy’s grandmother, mother and younger brother had been on the property at the time Gus disappeared.

“We don’t believe now that Gus is alive,” he said.

The person who was under suspicion was no longer co-operating with police, he said.

Fielke said police had uncovered discrepancies and inconsistencies in information provided relating to timelines and versions of events.

“As a result of these inconsistencies, and investigations into them, a person who resides at Oak Park Station has withdrawn their support for the police and is no longer cooperating with us,” he said.

On January 14 and 15, investigators from SA Police’s Taskforce Horizon executed a search warrant at Oak Park station and seized a car and motorcycle.

“You appreciate at this time I cannot make any further comment about the suspect, given that this is now a criminal investigation and a declared major crime,” Fielke said on Thursday.

“What I can say, however, is that we’ll continue to thoroughly and meticulously investigate the disappearance of Gus until we get an outcome.

“We’re all focused and determined to locate Gus and return him to his parents. Nothing is off the table as we work towards that outcome.”

Fielke said police had three theories from the beginning — that Gus had walked off from the remote property and got lost, that he was abducted or that someone known to him was involved in his disappearance and suspected death.

He spoke in great detail about massive searches that had been carried out, on the ground and from the air.

Fielke said the search for Gus had been “unprecedented”, involving hundreds of police, the Army, SES and Indigenous trackers over a vast ground area.

Oak Park station had been searched eight times and turned up no evidence that Gus had wandered off. A vast aerial operation had detected no trace of the child.

There was also no evidence that Gus had been abducted, he said, citing the property’s extreme remoteness in South Australia’s outback.

“Unless you know where you’re going and the way there, [Oak Park Station] can only be accessed by one of two 45-kilometre dirt roads that come from different directions,” he said.

“They can only be accessed by four-wheel drive. If you’re driving on those roads, you’re going to one of the stations or you’re lost. It’s not a place you just go to.

“There’s several gates on each of the driveways which impede access to Oak Park station, and a number of these gates are regularly locked and secured by the station owners and the property owners in the area.”

Fielke said police had spoken to and discounted “all people in the broader area” and from further afield who had a risk profile associated with abduction or child offending.

“We have identified and spoken to neighbours and people who were or had been working on Oak Park station or neighbouring properties at around the time of Gus’s disappearance,” he added.

“We have identified people and vehicles in the vicinity of or travelling around the area of Oak Park station at the time of Gus’s disappearance.

“All of those people have been contacted and discounted.”

No sign of Gus was found after a dam was drained. Photo: SA Police

The initial 10-day air and ground search for Gus was one of the largest SA Police had undertaken.

In November, search teams focused on six mine shafts near property, 40km south of Yunta, using specialist officers, but found no trace of Gus.

A four-day search within a 5.5km radius of the homestead in early-October also concluded without any evidence.

The family previously expressed their devastation over the boy’s disappearance in a September 30 statement.

“This has come as a shock to our family and friends, and we are struggling to comprehend what has happened,” it said.

“Gus’s absence is felt in all of us, and we miss him more than words can express.

“Our hearts are aching, and we are holding onto hope that he will be found and returned to us safely.”

-with AAP